Types of Thermodynamics Equilibrium

Thermodynamic Equilibrium

"A system will be in thermodynamic equilibrium with its surroundings when that system and its surroundings are simultaneously in thermal, mechanical and chemical equilibrium". 

Equilibrium is a term that refers to a steady state conditions (or static condition), the absence of change. In the state of equilibrium, no driving force is present so the properties of the system no longer change. The state of equilibrium can be achieved in two ways;

• When the system and the surrounding are not interacting.

• When the system and the surrounding are interacting.

☛ Thermodynamic equilibrium = Thermal equilibrium + Mechanical equilibrium + Chemical equilibrium. 

All the three put together is Thermodynamic equilibrium.

☛ Equilibrium is anything which is

a. Uniform throughout the system

b. Does not change with respect to time.

c. There is no tendency for spontaneous change.

☛ "For a single system, it is said to be in thermodynamic equilibrium if the state variables like Entropy and Internal Energy do not change".

Let us define the types of equilibrium based on the system's property in detail;

(a)Thermal Equilibrium

Definition Of Thermal Equilibrium??

"A system will be in thermal equilibrium with its surroundings when there is no difference in temperature between that system and its surroundings ,i.e., there is no net flow of Thermal energy between the system and the surroundings" (or when the temperature of the system is same as that of the surrounding then the system is said to be in thermal equilibrium with its surroundings). 

In thermal equilibrium, there is no heat transfer from the system to the surroundings as no temperature gradient exists between them. Let us understand it with an example-

Initially two gases, gas 'A' and gas 'B', are at different temperatures (say, 200°C &300°C). These two gases are separated by an adiabatic wall within a system. Now as soon as we remove this adiabatic wall, heat transfer will begin between these gases within the system. And the heat exchange will continue until the temperature of the two gases becomes the same(say, 250°C).

After achieving this stage, there will be no heat transfer between the bodies. This stage is called Thermal Equilibrium. This means that gas A and gas B are in thermal equilibrium with each other because both gases have the same temperature.

For Example-

1) Hot coffee becomes cold after some time. (Because the hot coffee is surrounded by the cold air of the room. This hot coffee loses its heat to the surrounding air of the room. Hot coffee will keep on losing the heat until its temperature becomes equal to the temperature of the surrounding air of the room. As a result, the example of a hot coffee cup illustrates the thermal equilibrium)

2) Vegetables and refrigerator air

(If you keep vegetables and fruits overnight in your refrigerator, then all the vegetables and fruits will cool down and their temperature will be the same as that of the temperature of the air inside the refrigerator. Thus, all the vegetables, fruits as well as surrounding air of the refrigerator are in thermal equilibrium with each other)

☛ "A single system is said to be in Thermal equilibrium if the temperature throughout the system is uniform and Temperature is not changing".

(b)Mechanical Equilibrium

Definition Of Mechanical Equilibrium??

"A system will be in mechanical equilibrium when no mechanical driving force of any kind exists between the system and its surroundings". For Example-

Pressure is a mechanical driving force (such as pressure, force, etc.), so when the pressure of the system is not changing with time, we can say that the system will be in mechanical equilibrium. Or we can say that the pressure remains the same at all points within the system. 

Let us understand it with an example-

Initially two gases, gas 'A' and gas 'B', are at different pressures. These two gases are separated by an insulated wall within a system. Now as soon as we remove this insulated wall, the high pressure gas expands into the low pressure gas within the system. And this diffusion of high pressure and low pressure gases will continue until the pressure of these two gases becomes the same. And as soon as the pressure of both the gases becomes equal, we can say that our system will be in mechanical equilibrium.

➝ "Gym Treadmill Is Best Example Of Mechanical Equilibrium". Because, the gym treadmill on which we run but we do not move forward because the gym treadmill pushes you backward with the same force you are trying to push forward. So, this is the best example of mechanical equilibrium.

(c)Chemical Equilibrium

"A system will be in chemical equilibrium when there is no change in the chemical composition (such as concentration) within the system over time and no chemical reaction takes place inside the system" (Or when there is no difference in concentration between the system and surrounding). 

A system will be in chemical equilibrium when no chemical driving force of any kind exists between the system and its surroundings. 

For Example- 

1) Water kept in a bottle is an example of chemical equilibrium.

(In water, the chemical composition remains H₂O only in the entire volume of water. Thus, water kept in a water bottle has the same chemical composition (H₂O). Thus, water kept in a bottle is an example of chemical equilibrium)

2) "Diffusion" is taking place when there is a difference in chemical potential (or in concentration) between system and surrounding (or between two systems). Here, the chemical potential is the driving force between the system and its surrounding(or between two systems). When this chemical potential of system is the same as that of the surrounding after some time then we can say that the system will be in chemical equilibrium with its surrounding. 

Let's understand it in a better way- 

Initially two gases, gas 'A' and gas 'B', are at different concentrations say C₁ & C₂. These two gases are separated by an insulated wall within a system. Now as soon as we remove this insulated wall, the higher concentration gas diffuses into the lower pressure gas within the system. And this diffusion of high concentration and low concentration gases will continue until the concentration of these two gases becomes the same ( say, C = C₁' = C₂'). And as soon as the concentration of both the gases becomes equal, we can say that our system will be in chemical equilibrium. 

➝ Since, in many applications of thermodynamics, chemical reactions are of no concern therefore if no chemical reaction takes place between the system and surrounding or within the system, the chemical composition such as concentration, chemical potential etc. is the same throughout the system. As a result, the system will be in chemical equilibrium with its surroundings.

➝ A system in which its chemical composition does not change with time and no chemical reaction takes place in it is said to be in chemical equilibrium.

Hope you have found this article helpful!!

Let me know what you think about TYPES OF THERMODYNAMICS EQUILIBRIUM. Feel free to comment if you have any queries.!!